1. Field of the Invention
The present invention relates to a biochip reader and, in particular, to a biochip reader using light from a coherent source such as laser as excitation light.
2. Description of the Prior Art
FIG. 1 is a schematic view illustrating the principle and configuration of a biochip reader. This figure is based on the confocal optical system, mentioned on pages 132 and 133 of “Application of confocal laser microscopes to medicine and biology,” New Optical Microscopes, Vol. II, published by Gakusai Kikaku (which means “Interdisciplinary Planning”) Co., Ltd. on Mar. 28, 1995. In a biochip reader based on such a schematic view illustrating its principle and configuration, laser light from light source 1 emitted through pinholes provided in pinhole plate 2 is incident to objective lens 5 after being collimated by lens 3 and transmitted through dichroic mirror 4. Objective lens 5 condenses this excitation light and irradiates sample (biochip) 6.
The fluorescent substances stuck to genes on biochip 6 emit fluorescence by being excited with excitation light. This fluorescence is reflected by dichroic mirror 4 after passing through objective lens 5 and then concentrated by condenser lens 7 and forms image on the plane of pinhole plate 8. These images are the fluorescence images of genes on sample 6 and are detected by detector 9.
To observe the fluorescence image of sample 6 surface (two-dimensional image), it is necessary to scan the sample 6 surface with excitation light. In this case, normally, the excitation light is not scanned but the stage on which sample 6 is placed (not shown in the figure) is scanned in the direction perpendicular to the optical axis (called stage scan).
In such a biochip reader, laser light is used as the light source because use of white light as a source causes a shortage of light quantity. In addition, employing the confocal type using pinholes not only prevents the detected image from being affected by dust stuck to the surface of sample 6, but also prevents speckle noise from being generated because the excitation light is emitted to the sample after being condensed.
However, in such conventional biochip readers, there are problems that adjustment or conditioning of pinholes is troublesome and also this raises its cost. Furthermore, it is disadvantageous that the stage is required to be durable because it is moved for scanning and thus the cost of the stage becomes high.
Next, FIG. 2 shows another example of a schematic view of a conventional biochip reader illustrating its principle and configuration. Such a principle is, for example, mentioned on page 158 of “GFP and Bio-imaging,” Experimental Chair in Post-genome Age, No. 3, a separate volume of Experimental Medicine, published by Yodosha Co., Ltd. on Oct. 25, 2000.
In FIG. 2, excitation light from a parallel-light light source such as laser (not shown in the figure) is concentrated by condenser lens 7 and incident to objective lens 13 after being reflected by dichroic mirror 12. In this case, the excitation light forms an image in the position of focal length f of objective lens 13 and this image acts as the second source to be incident to objective lens 13.
Sample 6 is irradiated with the excitation light that has transmitted through objective lens 13. As a sample, for example, a DNA chip that contains DNA on a slide glass whose surface is flat, or the like is used. At each site 15 of the DNA chip, fluorescent substances with which the DNA is labeled emit fluorescence by being excited with excitation light. The fluorescence forms an image on detector 18 via the image forming optical system. In other words, the fluorescence is made parallel by objective lens 13 as shown with continuous lines, passes through dichroic mirror 12 and barrier filter 16, and is incident to lens 17. The image of sample 6 formed by lens 17 is detected by detector 18.
In this case, the behavior of the excitation light irradiating over the entire sample surface is as follows:
Excitation light reflected at the flat sample surface shown with broken lines (this excitation light is called the reflected excitation light or the return excitation light) is focused by objective lens 13 and focused in the position of focal length f of objective lens 13.
The excitation light is incident to detector 18 after passing through lens 17 whose intermediate image plane is set at this focal position.
In addition, the reflected excitation light is transmitted through dichroic mirror 12 and barrier filter 16 before passing through lens 17. Barrier filter 16 is formed to pass fluorescence but reject (attenuate) the reflected excitation light and thus the reflected excitation light mixed into detector 18 as background light is reduced by being passed through this barrier filter 16.
However, in such conventional microscopes, the reflected excitation light cannot be sufficiently reduced although it is attenuated with the barrier filter. Although background light must be reduced to approximately 10−9 of the fluorescence to be detected in the measurement of fluorescent molecules or the like, there is a problem that an attenuation factor (ratio of exit light intensity to incident light intensity) of only about 10−7 can be obtained in this reader. Thus, the attenuation is clearly not sufficient.
Further, FIG. 3 is a schematic view of the biochip reader using a microlens array system illustrating its principle and configuration, mentioned in the Japanese patent application No. 2001-2264 submitted by the applicant concerned. In FIG. 3, a plurality of microlenses ML is arranged on microlens array 21, and light that has passed through each microlens (excitation light) is emitted to sample 23 through dichroic mirror 22. Sample 23 is a biochip, into each of whose cells genes are poured, each cell being arranged at the same pitch as the above microlenses and thus spatially arranged so that excitation light from each microlens irradiates each cell respectively.
Fluorescent substances are stuck to each gene on the biochip and generate emission owing to irradiation of excitation light. The emitted fluorescence is reflected by dichroic mirror 22, incident to lens 25 through barrier filter 24, and forms an image on detector 26 (e.g. a camera). In such a manner, a fluorescence image of the biochip can be observed with camera 26.
In addition, barrier filter 24 acts to transmit fluorescence but reject excitation light, and thus the use of this filter can prevent the excitation light reflected by the surface of sample 23 from being incident to camera 26.
However, in such biochip readers, if shading (cone-shaped light intensity distribution) is included in excitation light from the light source, non-uniformity is generated in read data. To prevent this, it is suggested to make the ratio a of the minimum value of light intensity to its maximum value to be 10 to 20%, by making the amount of shading small using only a center portion of the above conical intensity distribution as shown in FIG. 3. However, there occurs another problem that much light is wasted (the light-utilizing efficiency deteriorates) because this method discards light in the peripheral portion.
Furthermore, if the expression of mRNA in a biochip is to be measured using cDNA, there are large differences in the amounts of expression, which causes problems such as cases where a 10- to 100-fold difference exists as shown in FIG. 4(b) between the expression (signal intensity) of gene A and that of gene B shown in FIG. 4(a). That is, if the amount of expression is to be measured precisely without giving any change, analog-to-digital converters and amplifiers used in the detector must have wide dynamic ranges and high accuracy, and so are expensive. This is a problem.
In addition, there is another problem that, although there is a method to measure the amount of expression several times by changing the gains of analog-to-digital converters and amplifiers, this method takes time for measurement, and dispersion in measured values and discoloring of biochips also increase.